Jae-Chan Choi

Prediction of surface-fracture initiation in the axisymmetric extrusion and simple upsetting of an aluminum alloy

Abstract Most bulk metal forming processes may be limited by ductile fracture, such as an internal and surface fracture developing in the workpiece. It is important to predict the conditions within the deforming workpiece which may lead to fracture, as by using this prediction it may be possible to choose appropriate process conditions and to modify the forming processes to produce sound products. This paper suggests a scheme to simultaneously accomplish both the prediction of surface-fracture initiation and the analysis of deformation in the axisymmetric extusion and simple upsetting of an aluminum alloy. The Cockcroft-Latham criterion, which has been applied successfully to a variety of loading situations, is used in the present investigation to estimate if and where surface fracture will occur during the deformation process. The numerical predictions and experimental results for the two types of metal-forming process under consideration are compared. The proposed scheme successfully predicts the site of surface-fracture initiation and the corresponding level of deformation observed experimentally.

Finite-element simulation of the shear process using the element-kill method

Abstract The major objective of the present paper is to establish an analytical technique in order to closely understand and analyze the actual shearing process. First of all, isothermal and non-isothermal finite element (FE)-simulation of the shearing process are carried out using the finite-element software DEFORM. Based on preliminary simulation using DEFORM, the FE program to analyze the two-dimensional shearing process is developed. The ductile-fracture criterion and the element-kill method are used also to estimate if and where a fracture will occur and to investigate the features of the sheared surface in the shearing process. It can be seen that the developed program combined with the ductile fracture criterion and the element-kill method has enabled the achievement of FE-simulation from the initial stage to the final stage of the shearing process. The effects of the punch–die clearance on the shearing process are investigated also. In order to verify the effectiveness of the proposed technique, the simulation results are compared with known experimental data. The results of the present work are in close agreement with published experimental results.

Finite element analysis for the semi-solid state forming of aluminium alloy considering induction heating

Abstract The major objective of this study is to establish an analytical technique in order to investigate the behavior of semi-solid material considering induction heating of the workpiece. The induction heating process is analyzed using the commercial finite element software, ansys . The finite element program, sfac2d , for the simulation of deformation in the semi-solid state is developed in the present study. The behavior of semi-solid material is described by a viscoplastic model for the solid phase and by Darcy’s law for the liquid flow. Simple compression and closed-die compression processes considering induction heating are analyzed. To validate the effectiveness of the proposed analytical technique, the results of simulation are compared with those of experiment.

A new extrusion process for helical-gears: experimental study

Abstract A new process design for the cold extrusion of helical-gears (a modified Samantas gear-extrusion process) has been explored in this study. The process design involves the structure of the die, and the manufacture of the electro-discharge machining electrodes and the extrusion die and tooling. To design these electrodes, a computer software system was developed. The stress and elastic deformation of the dies during the extrusion process for both Samantas and Chois cold extrusion process were calculated using a commercial finite-element code. The accuracy of the extruded gears and of an extruded-shaved gear was measured. The results are summarized as follows: (1) gear electrodes were designed successfully using a CAD program. (2) The electrodes were machined with a standard or a shaving hob cutter by controlling the addendum modification factor or the shaving allowance. (3) The strength of the die teeth was improved remarkably using Chois new process. (4) The accuracy of the gears was not affected greatly by elastic deformation of the die. (5) The tolerance of the gears from the Choi-process were closer than those from the Samanta-process. Therefore, the Choi-process can be used as an advanced technique to replace the conventional hobbing process for gears.

Effect of caffeic acid phenethyl ester on Prevotella intermedia lipopolysaccharide‐induced production of proinflammatory mediators in murine macrophages

BACKGROUND AND OBJECTIVE Caffeic acid phenethyl ester (CAPE) has numerous potentially beneficial properties, including antioxidant, immunomodulatory and anti-inflammatory activities. However, the effect of CAPE on periodontal disease has not been studied before. This study was designed to investigate the efficacy of CAPE in ameliorating the production of proinflammatory mediators in macrophages activated by lipopolysaccharide (LPS) from Prevotella intermedia, a pathogen implicated in periodontal disease. MATERIAL AND METHODS LPS from P. intermedia ATCC 25611 was isolated by using the standard hot phenol-water method. Culture supernatants were assayed for nitric oxide (NO), interleukin (IL)-1β and IL-6. We used real-time polymerase chain reaction to quantify inducible NO synthase, IL-1β, IL-6, heme oxygenase (HO)-1 and suppressors of cytokine signaling (SOCS) 1 mRNA expression. HO-1 protein expression and levels of signaling proteins were assessed by immunoblot analysis. DNA-binding activities of NF-κB subunits were analyzed by using the enzyme-linked immunosorbent assay-based kits. RESULTS CAPE exerted significant inhibitory effects on P. intermedia LPS-induced production of NO, IL-1β and IL-6 as well as their mRNA expression in RAW264.7 cells. CAPE-induced HO-1 expression in cells activated with P. intermedia LPS, and selective inhibition of HO-1 activity by tin protoporphyrin IX attenuated the inhibitory effect of CAPE on LPS-induced NO production. CAPE did not interfere with IκB-α degradation induced by P. intermedia LPS. Instead, CAPE decreased nuclear translocation of NF-κB p65 and p50 subunits induced with LPS, and lessened LPS-induced p50 binding activity. Further, CAPE showed strong inhibitory effects on LPS-induced signal transducer and activator of transcription 1 and 3 phosphorylation. Besides, CAPE significantly elevated SOCS1 mRNA expression in P. intermedia LPS-stimulated cells. CONCLUSION Modulation of host response by CAPE may represent an attractive strategy towards the treatment of periodontal disease. In vivo studies are required to appraise the potential of CAPE further as an immunomodulator in the treatment of periodontal disease.

Finite element simulation and experiment for extrusion of semi-solid Al 2024

It is the objective of this study to analyze the effect of various process variables on the quality of extruded product and extrusion force for semi-solid extrusion of Al 2024 with solid phase structure of globular type by the finite element method. Process variables considered in the present study are initial solid fraction, ram speed, semi-angle of die and reduction in area. The FE-simulation code used in this study has been updated from the previous developed code. The flow and deformation of the semi-solid alloy are analyzed by coupling the deformation of the porous skeleton and the flow of liquid phase using Darcys law. It is assumed that the initial solid fraction is homogeneous. The results of the experiment are compared with those of simulation in order to verify the usefulness of the developed FE-simulation code.

Preform design in extrusion by the FEM and its experimental confirmation

Abstract An undesirable convex configuration at the front of an axisymmetric product occurs in the extrusion of a short bar, that is required to be removed by machining in order that the front of the product be flat. However, a flat front to a product can be obtained without machining by the use of an appropriate design of preform. Here the preform shape to produce a flat front after extrusion is designed using the rigid-plastic finite-element method and its backward-tracing scheme. Experiments into the extrusion process with the same conditions as those of the numerical simulations have been carried out to confirm the results relating to the numerical preform design, the results showing that the numerically designed preform is satisfactory in achieving the design purpose of a flat front the axisymmetric extrusion processes. The confirmation of the backward-tracing scheme of the finite-element method is valuable and necessary for wide application of the design scheme to preform design in practical forming processes.

An upper-bound analysis of the closed-die forging of spur gears

Abstract Closed-die forging of spur gears of which the shape of the fillet is a straight line in the radial direction is investigated by means of the upper-bound method. A kinematically admissible velocity field has been newly proposed in this paper: specifically an involute curve has been introduced to represent the tooth profile of the forging die. It was assumed that a constant frictional stress is applied on the contacting surfaces. Utilizing the formulated velocity field, numerical calculations has been carried out to investigate the effects of varying various parameters, such as modules, numbers of teeth and friction factors, on the forging of the spur gears. It was found that forging load or average relative pressure was dependent predominantly on the number of teeth.

Upper bound analysis for forging of trochoidal gears

Forging of trochoidal gears has been investigated by means of upper-bound analysis. A kinematically admissible velocity field for forging of a trochoidal gear was proposed. A neutral surface has been used to represent the inward flow of material during forging operation by using a hollow billet with a flat punch. By using the suggested kinematically admissible velocity fields, the applied forces were successfully calculated and compared with experimental inspections. The experimental set-up was installed in a 200-ton hydraulic press for forging. The billets of A1 2218 aluminum alloys were slightly coated with phosphate. It was found that the theoretical solutions were useful to predict the forging load forforging of trochoidal gears.